The digital image sensor, such as a charged coupled device (CCD), of a digital camera has a plurality of photo-sites arranged in a colored filtered array or pattern, such as a RGB Bayer pattern described in U.S. Pat. No. 3,971,065. In the RGB Bayer pattern, each photo-site is filtered so that it is receptive to either red, green, or blue or some variation thereof. The original bit-for-bit digital image file captured by the digital imaging sensor is referred to as a RAW file or RAW image. Depending on a number of variables, the RAW image may typically require 8 to 18 MB of memory space. The type of colored filter array and digital imaging sensor varies typically based on the manufacture of the digital camera. For example, some color filtered arrays use a pattern of yellow, cyan, green, and magenta.
Typically, the digital camera has an image pipeline that performs a demosaicing or de-Bayering process on the RAW image and transforms the image with a compressing algorithm to output a JPEG or other type of compressed file suitable for display and viewing. However, the RAW image captured by the digital camera can be uploaded to a computer, and computer software, such as Apple's Aperture 1.0, operating on the computer can allow a user to perform various manual operations on RAW image.
The color information of processed digital images can be characterized by a number of color models. One color model is the RGB color space, which uses combinations of red (R), green (G), and blue (B) to produce a plurality of colors. Some RGB color spaces used for digital cameras include Standard RGB (sRGB) and Adobe RGB. Another color model is the CIE XYZ color space created by the International Commission on Illumination (CIE) in 1931. Mathematical techniques can be used to transform color information from one color space to another.
Digital cameras have white balance settings, such as auto, incandescent, fluorescent, cloudy, sunny, or sensitivity (e.g., ISO 100, ISO 2000, etc). The settings are used to match the white balance of the digital camera to the color temperature of light from the illuminant that is illuminating the subject in the image. Characteristics of various standard illuminants are defined in the art. For example, illuminant A is used to represent incandescent lighting and is defined by the profile of a black body radiator at 2856 K. Illuminant series D is used to represent natural daylight. A number is used in the name of the D series illuminant to indicate the correlated color temperature (CCT) of the source. For example, illuminant D50 has a CCT of 5000 K, and illuminant D65has one of 6500 K. Illuminant series F is used to represents various types of fluorescent lighting. For example, illuminant F2 represents cool white fluorescent, while illuminant F11 represents a narrow-band fluorescent. When a digital image is obtained under one illuminant with a digital camera, it is possible to use a white point conversion technique to estimate the characteristics of the image under a different illuminant.
Various considerations must be addressed when processing a digital image obtained with a digital camera or other imaging device. One consideration involves preserving the spatial quality and the detail of the digital image, while another consideration involves sufficiently representing the color of the digital image. In many ways, these two considerations are interrelated.